1. Field of the Invention
The present invention relates to a synthetic fused silica member and a method for producing the same, as well as an optical member for excimer laser. In particular, the present invention relates to a synthetic fused silica member that can suitably be used for optical applications utilizing as a light source high energy ultraviolet rays such as those from excimer lasers, typically KrF and ArF excimer lasers, for example, use as a lens material for steppers and a synthetic fused silica material for photomasks, and a method for producing such a synthetic fused silica member, as well as an optical member for excimer laser produced from the synthetic fused silica material such as lenses, mirrors and photomasks.
2. Description of Related Art
With recent use of higher integration degree of semiconductor devices, in particular, VLSIs, the circuit pattern has rapidly become finer. Therefore, the wavelength and illuminance of light sources for the VLSI production are becoming shorter and stronger, respectively.
On the other hand, synthetic fused silica, which shows good performance in the ultraviolet range, has been getting importance as not only a material for photomasks but also a material for optical members such as lenses and mirrors in optical systems of light exposure apparatuses.
Such synthetic fused silica members are produced by, for example, vaporizing a raw material silane compound, hydrolyzing or oxidizing by combustion the vaporized silane compound in oxyhydrogen flame to form silica microparticles so that the silica microparticles should deposit on a rotating refractory carrier, and melting the silica microparticles during the deposition.
However, when a synthetic fused silica member produced from such a material is irradiated by a high energy ultraviolet light such as those from excimer lasers, it may emit yellow fluorescence at a wavelength of 560-580 nm. Therefore, when such a member is used for optical applications utilizing ultraviolet lights such as those from excimer lasers as a light source, for example, used as a lens for stepper, a material of photomask substrate or the like, it may cause a problem of light exposure of resists and the like. Accordingly, it may be unsuitable for use as a lens of stepper or the material of photomask substrate.
The present invention has been accomplished from the viewpoint of the above technical circumstance, and its object is to provide a synthetic fused silica member that does not emit yellow fluorescence at a wavelength of 560-580 nm even when irradiated with an ultraviolet light by an excimer laser or the like, and therefore can be suitably used for optical applications utilizing ultraviolet lights such as those from excimer lasers as a light source, for example, use as a lens material for stepper or the like. It is also an object of the present invention to provide a method for producing the aforementioned synthetic fused silica member, and an optical member for excimer laser produced from the aforementioned synthetic fused silica member, for example, a lens, photomask and the like.
The inventors of the present invention earnestly conducted studies in order to achieve the aforementioned object. As a result, it was found that sulfur impurities such as SCl2 and S2Cl2 contained in the raw material silane compound caused the yellow fluorescence emission at a wavelength 560-580 nm of the synthetic fused silica member upon ultraviolet irradiation. That is, such sulfur impurities contained in the raw material silane compound are simultaneously vaporized when the silane compound is vaporized, and introduced into the oxyhydrogen flame together with the silane compound gas. In this case, the sulfur impurities are taken into the synthetic fused silica, which is considered to be inherently likely to incorporate sulfur compounds due to its structural characteristics. As a result, the synthetic fused silica member emits yellow fluorescence at a wavelength of 560-580 nm upon ultraviolet irradiation. The inventors of the present invention accomplished the present invention based on this finding.
That is, the present invention provides a method for producing a synthetic fused silica member comprising vaporizing a raw material silane compound, hydrolyzing or oxidizing by combustion the vaporized silane compound in oxyhydrogen flame to form silica microparticles so that the silica microparticles should deposit on a rotating refractory carrier, and melting the silica microparticles during the deposition to form the synthetic fused silica member, characterized in that sulfur impurities are preliminarily removed from the raw material.
If the causative substances of the aforementioned yellow fluorescence, i.e., the sulfur impurities, have been preliminarily removed from the raw material as in the aforementioned method of the present invention, the sulfur impurities are not introduced into the oxyhydrogen flame together with the silane compound when the silane compound is vaporized. Therefore, the sulfur impurities are not introduced into the produced synthetic fused silica member. As a result, the obtained synthetic fused silica member does not emit yellow fluorescence at a wavelength 560-580 nm even when irradiated with an ultraviolet light by an excimer laser or the like.
The influence of sulfur impurities contained in the raw material silane compound on the synthetic fused silica member has not been considered at all so far. Therefore, the sulfur impurities have not been preliminarily removed from the material in any conventional methods.
In the aforementioned method for producing a synthetic fused silica member according to the present invention, the sulfur impurities are preliminarily removed from the raw material preferably to a level of 10 ppm or less.
If the sulfur impurities are preliminarily removed from the material to a level of 10 ppm or less, a significant amount of sulfur impurities that may cause the problem are not introduced into the synthetic fused silica member, and therefore the synthetic fused silica member does not emit yellow fluorescence at a wavelength 560-580 nm even when irradiated with an ultraviolet light by an excimer laser or the like.
In the method for producing a synthetic fused silica member according to the present invention, the sulfur impurities are preferably sulfur chlorides.
Among the sulfur impurities, sulfur chlorides (e.g., SCl2, S2Cl2), in particular, have a boiling point substantially the same as that of the raw material silane compound. Therefore, among the sulfur impurities, sulfur chlorides are particularly likely to be simultaneously vaporized when the silane compound is vaporized, and introduced into oxyhydrogen flame together with the silane compound. That is, sulfur chlorides are more likely to be incorporated into the synthetic fused silica members compared with other sulfur impurities. Therefore, since sulfur chlorides among the sulfur impurities are particularly likely to cause the yellow fluorescence, it is particularly necessary to preliminarily remove sulfur chlorides.
In the aforementioned method according to the present invention, the silane compound is preferably a compound represented by the formula (1):
RnSiX4xe2x88x92nxe2x80x83xe2x80x83(1)
wherein R each represents hydrogen atom or a monovalent aliphatic hydrocarbon group, which may be identical or different, x represents a halogen atom or a group xe2x80x94OR, and n represents an integer of 0-3, or the formula (2):
SixRyOzxe2x80x83xe2x80x83(2)
wherein R has the same meaning as defined above, x represents an integer not less than 2, y represents a positive integer not more than 2x+2, and z represents a positive integer not more than 2x.
Those silane compounds represented by the aforementioned formulae can easily be obtained as a usual material of synthetic fused silica members, and have widely been used. Therefore, they are preferably used also in the present invention.
The present invention also provides a synthetic fused silica member produced by the aforementioned method for producing a synthetic fused silica member of the present invention.
Because the synthetic fused silica member produced by the method for producing a synthetic fused silica member of the present invention contains few or substantially no sulfur impurities, it does not emit yellow fluorescence at a wavelength 560-580 nm even when irradiated with an ultraviolet light by an excimer laser or the like.
The present invention further provides an optical member for excimer laser characterized in that it is produced from the aforementioned synthetic fused silica member of the present invention.
As described above, if the synthetic fused silica member of the present invention, which contains substantially no sulfur impurities, is used for the production of an optical member for excimer laser such as a lens and photomask, the optical member does not emit yellow fluorescence at a wavelength 560-580 nm even when irradiated with an ultraviolet light. Therefore, the optical member for excimer laser of the present invention is an extremely excellent optical member that does not cause light exposure of resists and the like due to the yellow fluorescence.
According to the production method of the present invention, a synthetic fused silica member containing few or substantially no sulfur impurities can be produced. As a result, this synthetic fused silica member does not emit yellow fluorescence at a wavelength of 560-580 nm even when it is irradiated with an ultraviolet ray of high energy by an excimer laser or the like. Therefore, it does not cause light exposure of resists or the like due to such yellow fluorescence. Accordingly, the synthetic fused silica member of the present invention is suitably used as a material for optical applications utilizing ultraviolet rays such as those from excimer lasers as a light source, for example, use as a lens for stepper, photomask and the like.